The present invention relates to structural assemblies and, more particularly, relates to the application of friction stir welding to superplastically formed structural assemblies.
Superplastic forming (xe2x80x9cSPFxe2x80x9d) is a process used to form structural assemblies having complex three-dimensional shapes, such as the two- and three-sheet assemblies 10, 11 illustrated in FIGS. 1A and 1B, respectively. These assemblies are formed from metal alloys, such as aluminum and titanium alloys (particularly Zn-22Al and Ti-6Al-4V) that exhibit superplastic behavior at certain temperatures, i.e., large elongation (up to 2000 percent) under low strain rates. During the SPF process, a multi-sheet SPF pack is placed into a shaping die and heated to a sufficiently high temperature to soften the sheets of material. Pressurized heated gas is then injected into the SPF pack, causing the pack to inflate and fill the die. The assembly is then cooled and removed from the die and final machining steps are performed, such as edge trimming, to form the finished structural assembly.
As illustrated in FIG. 2A, the SPF pack 12 used to form the structural assembly is constructed by stacking two or more sheets 13 of material (a three-sheet SPF pack is illustrated in FIG. 2) and joining the sheets by forming partial-penetration weld joints 14 making a pre-selected pattern using any conventional fusion welding processes such as oxyfuel, arc, and resistance welding. A partial-penetration weld joint joins two or more adjacent sheets in a stack, but generally does not join all the sheets in the stack. The partial-penetration weld joints define areas therebetween where the adjacent sheets 13 remain in contact after the SPF pack 12 has been inflated to form the structural assembly. As illustrated in FIG. 2B, prior to inflating the SPF pack 12, the sheets of material 13 in the stack are joined by full-penetration weld joints 16 along the periphery of the stack to thereby form a sealed pack 12. Plumbing fittings 17 are connected to the interior of the pack 12 through gas passages (not shown) machined into or between the sheets of material so that pressurized heated gas can be injected into the pack. The SPF pack 12 is typically sealed around the plumbing fittings 17 by fillet welds formed between the fittings 17 and the edge of the pack 12 using conventional fusion welding processes.
The SPF process is particularly advantageous since complex shapes can be formed with lower tooling costs. Additionally, structural assemblies formed using the SPF process have minimal residual stresses. Notwithstanding these benefits, the materials used during the SPF process are generally limited to those that are readily weldable using conventional fusion welding techniques, such as oxyfuel, arc, and resistance welding, due to the necessity of forming partial-penetration and full-penetration welds in preparing the SPF packs. Thus, xe2x80x9cunweldablexe2x80x9d materials are unavailable to designers for use with the SPF process, as these materials produce relatively weak weld joints. xe2x80x9cUnweldablexe2x80x9d materials are materials that possess high conductivity and quickly dissipate heat away from the weld joint and/or that exhibit cracking along the weld joint as a result of stresses caused by thermal expansion. Such materials include aluminum and some aluminum alloys, particularly some AA 2000 and 7000 series alloys. The exclusion of these materials from use with the SPF process has been problematic, as many of these materials possess special corrosion, fatigue, strength, density or ductility characteristics that are desired in certain applications.
In seeking better methods for forming SPF packs and, in particular, forming the partial-penetration and full-penetration welds between the individual sheets in the pack, a relatively new welding process known as friction stir welding has been proposed. As illustrated in FIGS. 3 and 3A, friction stir welding is a solid state process in which the probe 18 of a rotating friction stir welding tool 15, which is attached to a friction stir welding machine (not shown), is forced into or between workpieces 19 that are to be joined. The frictional heat generated by the rotating probe 18 and the shoulder 15a of the friction stir welding tool 15 creates a plasticized region or joint between the workpieces 19 that subsequently solidifies thereby joining the workpieces. See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion of friction stir welding, the contents of which are incorporated herein by reference.
Although friction stir welding is a solid state process that can be used to join materials that were previously considered unweldable using conventional fusion welding techniques, the use of friction stir welding to form weld joints between stacked sheets of material during the construction of SPF packs presents several problems. First, as illustrated in FIG. 4, the frictional heat conducted to the interface between the sheets 20 by the rotating friction stir welding probe 18 and the tool shoulder 15a, when combined with the pressure exerted on the sheets by the shoulder, can cause thermo-compression welding 21 of the interface between the adjacent sheets resulting in weld joints as wide as the diameter D of the shoulder. In this regard, the diameter D can vary, depending on the thickness of the material being welding, from approximately 0.2 inches to approximately 1 inch, and even as much as approximately 1.6 inches for relatively thick sheets. Thermo-compression welding 21 is particularly a problem when friction stir welding thin sheets of material, on the order of 1.5 mm in thickness. Ideally, in order to maintain the tolerances of the finished structural assembly and minimize stock material usage, the weld joints should only be as wide as the diameter P of the friction stir welding probe 18, which typically is about as large as the thickness of the plate or plates to be welded. For example, for a 1.5 mm plate, a 1.5 mm diameter probe would be acceptable. Secondly, as illustrated in FIG. 2B, to contain the pressurized heated gas that is injected into the SPF pack 12 during the SPF process, the pack is sealed by forming full-penetration weld joints 16 around the periphery of the pack. However, on the side of the SPF pack 12 where the plumbing fittings 17 are attached, friction stir welding cannot be used as the rotating probe 18 will impinge upon and damage the plumbing fittings and/or obstruct the internal passages intended for delivery of gas to the interior portion of the SPF pack.
Thus, there is a need for improved methods of forming SPF packs, and particularly, for friction stir welding SPF packs. Such manufacturing methods should be cost effective, minimize thermo-compression welding of the interface between adjacent sheets of material and prevent damage to the plumbing fittings of SPF packs.
The present invention provides a superplastically formed structural assembly and an associated method for manufacturing. The structural assembly includes first and second structural members having facing surfaces. The first and second structural members can include a first outer structural member, a second outer structural member or one or more intermediate structural members. In one embodiment, the first and second structural members include first and second outer structural members. In another embodiment, the first and second structural members include first and second intermediate structural members. The first and second structural members may be formed of titanium, aluminum, or alloys thereof. In one embodiment, the first and second structural members are formed of dissimilar metals.
The structural assembly includes at least one friction stir weld joint joining the first and second structural members. The structural assembly may include a plurality of friction stir weld joints joining the first and second structural members. In one embodiment, the plurality of friction stir weld joints define areas therebetween wherein the facing surface of the first structural member is spaced apart from the facing surface of the second structural member. In another embodiment, the facing surface of the first structural member is at least partially covered with oxide. In yet another embodiment, the facing surface of the second structural member is at least partially covered with oxide. In still another embodiment, the oxide has a thickness of at least 5 nm. Advantageously, the oxide prevents thermo-compression welding of the first and second structural members adjacent the at least one friction stir weld joint.
The present invention also provides a method for manufacturing a structural assembly. In one embodiment, the method includes the steps of providing first and second structural members. Advantageously, a surface of at least one of the first and second structural members is selectively anodized to thereby prevent thermo-compression welding between the first and second structural members. In one embodiment, the selective anodizing step includes immersing the at least one of the first and second structural members in an anodize bath as the anode in an electrolytic cell. In another embodiment, the selective anodizing step includes brush anodizing the surface of the at least one of the first and second structural members. The first and second structural members are stacked and then selectively joined to form a sealed forming pack. The sealed forming pack is then superplastically formed to thereby form the structural assembly. The structural assembly may be machined after the superplastic-forming step.
In one embodiment, the superplastic-forming step includes positioning the sealed forming pack in a shaping die. The sealed forming pack is then heated according to a predetermined temperature schedule. Following the heating step, pressurized heated gas is injected into the sealed forming pack to inflate the sealed forming pack into a shape defined by the shaping die.
In another embodiment, the method of manufacturing includes the steps of selectively anodizing a surface of at least one structural member, which structural member can include a first outer structural member, a second outer structural member, or an intermediate structural member, to thereby prevent thermo-compression welding to the at least one structural member having the selectively anodized surface. In one embodiment, the selective anodizing step includes immersing the at least one structural member in an anodize bath as the anode in an electrolytic cell. In another embodiment, the selective anodizing step includes brush anodizing the surface of the at least one structural member. The first and second outer structural members and at least one intermediate structural member are then stacked such that the at least one intermediate structural member is positioned between the first and second outer structural members. The first and second outer structural members are then selectively joined to the at least one intermediate structural member to form a sealed forming pack. In one embodiment, the selective joining step occurs concurrently with the stacking step. The sealed forming pack is then superplastically formed to thereby form a structural assembly. The structural assembly may be machined after the superplastic-forming step.
In yet another embodiment, the method of manufacturing a structural assembly includes the steps of joining strips of fusion weldable material to corresponding first edges of first and second structural members. The first and second structural members are then stacked such that the strips of fusion weldable material attached to the corresponding first edges of the first and second structural members are superimposed. In one embodiment, at least one partial-penetration friction stir weld joint is formed between the first and second structural members after the stacking step. Prior to the partial-penetration-weld-joint-forming step, the surface of at least one of the first and second structural members may be selectively anodized. In one embodiment, the selective anodizing step includes immersing the at least one of the first and second structural members in an anodize bath as the anode in an electrolytic cell. In another embodiment, the selective anodizing step includes brush anodizing the surface of the at least one of the first and second structural members. After the stacking step, full-penetration friction stir weld joints are formed along a portion of the peripheral edges of the first and second structural members and the strips of fusion weldable material to thereby define a non-welded plumbing edge along the strips of fusion weldable material. Plumbing fittings are at least partially inserted into the plumbing edge of the strips of fusion weldable material. In one embodiment, prior to the stacking step, cutouts to receive the plumbing fittings can be machined in the plumbing edge of at least one of the strips of fusion weldable material. After the inserting step, a fusion weld joint is formed along at least a portion of the plumbing edge of the strips of fusion weldable material such that the plumbing fittings are sealed between the strips of fusion weldable material. The first and second structural members are then superplastically formed to thereby form the structural assembly. The strips of fusion weldable material attached to the corresponding first edges of the first and second structural members can be machined away after the superplastic-forming step.
The method of manufacture according to the previous embodiment may also include joining a strip of fusion weldable material to a corresponding first edge of at least one intermediate structural member. The first and second outer structural members and the at least one intermediate structural member are then stacked such that the at least one intermediate structural member is positioned between the first and second outer structural members and the corresponding strips of fusion weldable material are superimposed. In one embodiment, the surface of at least one structural member, which structural member may include the first outer structural member, the second outer structural member, or an intermediate structural member, may be selectively anodized prior to the stacking step. In one embodiment, the selective anodizing step includes immersing the at least one structural member in an anodize bath as the anode in an electrolytic cell. In another embodiment, the selective anodizing step comprises brush anodizing the surface of the at least one structural member. At least one partial-penetration friction stir weld joint may then be formed between the at least one structural member having a selectively anodized surface and an adjacent structural member concurrently with the stacking step. After the stacking step, full-penetration friction stir weld joints are formed along a portion of the peripheral edges of the first and second outer structural members, the at least one intermediate structural member, and the strips of fusion weldable material to thereby define a non-welded plumbing edge along the strips of fusion weldable material. Plumbing fittings are then at least partially inserted into the plumbing edge of at least one of the strips of fusion weldable material. In one embodiment, prior to the stacking step, cutouts to receive the plumbing fittings can be machined in the plumbing edge of at least one of the strips of fusion weldable material. After the inserting step, a fusion weld joint is formed along at least a portion of the plumbing edge of the strips of fusion weldable material such that the plumbing fittings are sealed between the strips of fusion weldable material. The first and second outer structural members and the at least one intermediate structural member are then superplastically formed to thereby form the structural assembly. The strips of fusion weldable material friction stir welded to the corresponding first edges of the first and second outer structural members and the at least one intermediate structural member may be machined away after the superplastic-forming step.
In yet another embodiment, the method of manufacturing a structural assembly includes the steps of drilling at least one aperture into a peripheral edge of at least one of a plurality of structural members, wherein the at least one aperture defines a primary gas passage. The plurality of structural members are stacked. After the stacking step, full penetration friction stir weld joints are formed along peripheral edges of the plurality of structural members other than a non-welded plumbing edge, wherein the plumbing edge comprises the primary gas passage. An edge member is secured to the plumbing edge of the plurality of structural members. The plumbing edge may be machined flush prior to the securing step. At least one aperture is drilled through the edge member to thereby define a secondary gas passage such that the secondary gas passage of the edge member is in fluid communication with the primary gas passage of the plumbing edge. A plumbing fitting is attached to the secondary gas passage of the edge member. The plurality of structural members are then superplastically formed to thereby form the structural assembly. The edge member may be machined away after the superplastic-forming step. In one embodiment, the surface of at least one of the plurality of structural members is selectively anodized prior to the stacking step. In one embodiment, the selective anodizing step includes immersing the at least one of the plurality of structural members in an anodize bath as the anode in an electrolytic cell. In another embodiment, the selective anodizing step comprises brush anodizing the surface of the at least one of the plurality of structural members. In another embodiment, at least one partial-penetration friction stir weld joint may be formed between the at least one structural member having a selectively anodized surface and an adjacent structural member after the selective anodizing step.
Accordingly, there has been provided a structural assembly and an associated method of manufacture allowing for the cost-effective manufacture of superplastically-formed structural assemblies using friction stir welding to form the SPF packs. The method of manufacture minimizes thermo-compression welding of the interface between adjacent sheets of material and prevents damage to the plumbing fittings of SPF packs. The resultant structural assemblies have fine details, close tolerances, and minimal residual stresses.